A current access network provides an access to Internet services mainly through ADSL or VDSL which uses a telephone line, or a cable modem which uses a coaxial cable. Both the telephone line and the coaxial cable are provided with copper wires. A bandwidth to the subscribers may vary depending on transmission distances and thus a maximum value of the bandwidth is limited to approximately several 10 Mb/s. However, demand on a wider bandwidth in an access network is rapidly increased as voice and text oriented services have evolved to data and video based services. As one method to meet this high speed requirement, a study on PON technologies based on optical fiber has actively carried out and a WDM-PON among current PON technologies, which can provide a bandwidth of more than 100 Mb/s and guarantee a high quality of service (QoS) to the subscribers, is perceived to be an ultimate solution. Generally, in the architecture of WDM-PON, AWG is mainly used as a filter for a wavelength division multiplexed signal. However, a wavelength assigned to per each subscriber brings about inventory problem and its variation depending on ambient temperature requires an expensive control unit. Thus, a low-cost light source having wavelength-independent operation, i.e., color-free operation is essentially required to easily control and manage wavelength.
Regarding a low-cost light source, Hyun-Deok Kim, et al., proposed a Wavelength-Locked F-P LD as a light source for WDM-PON in their article entitled “A low-cost WDM source with an incoherent light injected Fabry-Perot semiconductor laser diode,” IEEE Photon. Technol. Lett., vol. 12, no. 8, pp. 1067-1069, August 2000. The Wavelength-Locked Fabry-Perot laser diode proposed by Hyun-Deok Kim, et al., uses a method for locking an oscillation wavelength of F-P LD into a wavelength of a BLS by injecting the BLS into the F-P LD which oscillates in a multi-mode. This has currently been recognized to be the most cost-effective light source for WDM-PON. Further, a study on a wavelength-seeded reflective semiconductor optical amplifier (RSOA) using RSOA instead of F-P LD has also carried out actively.
In a wavelength-locked F-P LD or a wavelength-seeded RSOA, light emitting diode (LED) with a high output power, Erbium-Doped Fiber Amplifier (EDFA) emitting Amplified Spontaneous Emission (ASE), Super Luminescent Diode (SLD), etc. are used as a BLS being injected. However, in the ASE-based BLS being injected generally for wavelength-locked F-P LD, the filtered bandwidth of the BLS is determined by the bandwidth of AWG to be used. Accordingly, the narrower bandwidth of AWG leads to worse relative intensity noise (RIN) of the BLS, which results in a certain limitation in increasing a number of subscribers and a bit rate. More specifically, a channel spacing of AWG must be decreased to increase a number of subscribers. In such a case, however, the bandwidth of each channel is also decreased, which leads to worse RIN of the filtered BLS.
In addition, in case that a center wavelength of the BLS being injected corresponds to an oscillation wavelength of F-P LD, an effect of suppression in mode partition noise of F-P LD can be obtained. However, in case that the center wavelength of BLS lies in between two oscillation modes of F-P LD, the mode partition noise suppression effect cannot be obtained. Rather, the noise of wavelength-locked F-P LD is more increased than the RIN of the injected BLS. The noise is increased further, when the output of the wavelength locked F-P LD passes through AWGs for multiplexing and de-multiplexing.
As described above, the increased noise functions as the most serious obstacle in embodying wavelength independence of wavelength-locked F-P LD for a WDM light source as the channel spacing of AWG is narrower and the transmission data rate is higher. Accordingly, due to the above obstacle, there is a problem which cannot fully guarantee the noise characteristic which equals to or is below RIN requirement for satisfying a bit error rate (BER) of 10−12 (Q=7, where Q is one parameter having a 1:1 relationship with BER) in order to embody wavelength independence.